I was thinking about picking up the E39 M5 that was posted here a few months ago (the one with the caramel interior
). But I came back to reality after I did a bit of research and found out that it would get the same fuel economy as my full-size pickup ($100 fill-ups!). Admittedly, you can get around the fuel costs by having another DD (saving the M5 as a garage queen, which doesn't seem like a bad idea), but the real deal killer for me was the engine carbonization problem and the costly remedy. AFAIK, there is no guaranteed way to prevent the problem or to repair it without incurring labour-intensive dismantling.